The investigators' hypothesis is that add-on varenicline will be effective (versus placebo) in initiating abstinence from smoking in subjects with stable, euthymic bipolar disorder who are motivated to quit smoking within four weeks. This primary outcome will be assessed from randomization to 12 weeks or end of the treatment phase of the study. Secondarily, the investigators also hypothesize that varenicline will prevent relapse in the subsequent 12-weeks follow-up non-treatment phase. Furthermore, the investigators plan to test the effectiveness of varenicline in reducing nicotine withdrawal symptoms or urges to smoke, as well as its safety for use in stable bipolar patients when used as an add-on treatment for smoking cessation.